Device to convert mechanical vibrations into electrical oscillations



y 1941. R. VERMEULEN ETAL 2,240,918

DEVICE TO CONVERT MECHANICAL VIBRATIONS INTO ELECTRICAL OSCILLATIONS Filed June 18, 1938 INVENTOR-S Roelof VermeuLen Hermanus tefar'zu'al 'iacha ATTORNEY Patented May 6, 1941 DEVICE T CONVERT MECHANICAL VIBRA- TIONS INTO ELECTRICAL OSCILLATIONS Roelof Vermeulen and Hermanus Stefanus Fuchs, Eindhoven, Netherlands, assignors, by mesne as-. signments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 18, 1938, Serial No. 214,608 In Germany July 28, 1937 8 Claims. (Cl. 179100.41)

This invention relates to an electro-mechanical vibration converter for transforming me chanical vibrations into electrical oscillations.

It is known to use electrodynami-c vibration converters such as pick-ups in which the mechanical vibrations act on a coil form carrying a plurality of wire windings and adapted to move in a magnetic field. However light their construction may be, such coil forms provided. with wire always have a mass which is not negligible and which, particularly in the case of pickups for talking machine records, in which the scanning needle also has to move the comparatively heavy coil with it, involves the disadvantage that the needle point is subject to high forces and is readily worn. In practice, the life of the records is consequently also comparatively short.

According to the present invention, this disadvantage is substantially obviated by movably arranging a conducting rodlike-shaped conductor supported at at least two points within a magnetic field so as to oscillate like a rod between the bearing points.

The term rodlike is used here to express that the conductor is constructed and clampedin free from stresses at the bearing points in such manner that its rigidity plays an important part in determining the natural frequencies. A suitable choice of the dimensions, and the material of the conductor permits of ensuring in this manner that the natural frequencies become located on a very high level although the rigidity at the point at which the mechanical forces to be converted into electrical tensions act is very low. In the case of sound reproduce tion this means that much lower forces at the needle point than hitherto sufiice to obtain a suniciently large conductor amplitude.

Due to the very light and almost massless construction of the moving parts the needle of pick-ups formed in accordance with the present invention is only subject to low forces so that the pressure at which the needle engages the record may be slight. This results in small wear of the record and the needle. Such pickups can therefore be provided with advantage with a fixed uninterchangeable needle so that also the needle carrier and clamping device may be omitted. This ensures a further reduction in mass of the moving parts so that even very high frequencies, for example 8000 cycles/sec. and more, can be reproduced faithfully by such a vibration converter.

Oscillation converters similar to a certain extent to the vibration converters according to this invention but serving for the transformation of electrical oscillations into mechanical vibrations are already known per se, for example mirror oscillographs comprising a wire loop, said wire loop having a current passing through it and being thus moved like a string within a magnetic field. The invention discloses, however, the novel surprising fact that systems of the kind referred to can also be used in those cases, for example in pick-ups, where the conductors are set into vibration by mechanical agency and produce electric currents.

According to the present invention, the conductor which is supported at two points and which is preferably shaped in the form of a band, is preferably supported rigidly at one of its ends and is set into vibration at its other end which is extended through one of the bearing points on which it is arranged so as to be adapted to move but not to slide in a transverse direction.

If the conductor is shaped in the form of a band it presents the advantage of being capable of readily oscillating normally to the direction of width but of being rigid in the direction of width.

The movably supported end has the needle, in the, case of pick-ups, or the diaphragm, in the case of microphones, fixed to it, it being possible for the space between the bearing points to be closedby an uninterrupted envelope so as to be dust-proof. The two bearing points preferably serve as points for receiving current.

holder that the invention may be clearly understood and readily carried into effect, the above mentioned features and other features will be described more fully, by way of example, with reference to the accompanying drawing, in which Fig. 1 is a longitudinal section of a pick-up according to the invention.

Fig. 2 is a cross-sectional view of the said pick-up.

Fig. 3 shows in exploded perspective the arrangement of the various elements.

Referring to the drawing in greater detail, l0 designates a cylindrical casing of magnetic material, for instance iron, having connected to it by soldering magnetized pole pieces II and I2 made of magnet steel of high value, for instance a BHmax value of more than 1,500,000 'gauss. The casing I0 is surrounded by a sheath of insulating material l3 and is provided at its lower and upper surfaces with rings l4 and I 5 which are also formed of insulating material so that the upper and lower covers 5 and I! of electrically conductive material, for example aluminium or brass, which are forced onto the coated casing I6 by means of recesses i8 and I9, re spectively, formed in them, are electrically insulated from the casing Iii.

The upper cover i8 is closed entirely except for a small bore 29 which is concentrated with the recess 18 and serves for receiving the conductor. The lower cover I! is provided concentrically with the recess It with a collar 2! bearing on the insulating ring I A. of the casing It] and provided with a thread 22. A ring 23 can be screwed into this thread 22.

A thin copper diaphragm 2 3, for example 0.02 mm. thick, is clamped in stressed condition between the collar Zl and the ring 23. tioned above, a conductor 25, which is preferably formed of aluminium, has one of its ends s01- dered to the cover it; in the bore 28 and passes through the diaphragm 24; adjacent its other end and is soldered to this diaphragm and is thus capable of rotating at this point to a limited extent but is supported so as to be'prevented from sliding in a transverse direction. The conductor is provided with a tungsten point or a sapphire 28 at this end. This end, which extendsfrom the diaphragm, constitutes the scanning needle whosepoint 26 passes in the grooves of the record and thus performs vibrations normal to the plane of the drawing. For the purpose of attenuating the movements of the conductor 25 it is possible to pour a damping liquid such, for example, as glycerine or oil on to the diaphragm it.

The covers 56 and it are insulated from the casing it and from one another by means of the insulatingsheath i 3 and the insulating rings l4 and IE but each of them is connected electrically to the ends of the conductor 25 so'that the alternating voltage produced in this conductor 25 lies between the said covers. This alternating voltage is transmitted to an amplifying system (not shown) by means of two copper rods 2'5 and 28 which are rigidly connected to the covers it and H, respectively, by means of threads 29 and 33, respectively, and. which also serve jointly as a conducting or supporting arm. The rods are spaced apart and united to form. a mechanical unitary piece by means of a strap 3! of insulating material. The rods may be insulated themselves.

'It has been found possible toconstruct pickups of the kind described above in which the band-shaped armature has a weight of only about mg., the weight of the entire pick-up without conducting arms being about g. The rigidity of the armature when the latter has been put in place is about 6 g. per 0.1 mm. of

As menarmature deviation and this permits of causing the pick-up to engage the record at a pressure of about 10 g.

What we claim is:

1. An electrical pickup device comprising means providing a magnetic circuit and including an air gap, a conductive element in said air gap, and a pair of conductive members supporting said element at spaced points therealong for movement in said air gap, said element having electrical connection with said members and extending through one of said members whereby to provide an extension adapted to cooperate with a phonograph record.

2. The invention set forth in claim 1 characterized in that said last-named member is constituted by a flexible diaphragm.

3. The invention set forth in claim 1 characterized in that said last-named member is constituted by a flexible diaphragm and the other of said members is rigid, and characterized further in that said element extends through said diaphragm.

4. The invention set forth in claim 1 characterized in that saidelement is constituted by a metallic band and is arranged to normally'oscillate in the direction of width of said band.

5. The invention set forth in claim 1 characterized in that said magnetic circuit providing means includes a normally open-ended casing, and characterized further in that said conductive members close off the endsof said casing.

6. The invention set fourth in claim 1 characterized in that said magnetic circuit providing means includes a normally open-ended casing, characterized further in that said conductive members close oif the ends of'said casing, and characterized still further in that said element is axially aligned with said members.

'7. The invention set forth in claim 1 characterized in that said magnetic-circuit providing means is constituted by a casing of magnetic material having a relatively lowmagnetic value and a pair of pole pieces soldered to the inner Wall thereof, said pole pieces being formed -of magnetic material of relatively high value.

8. An electrical pickup device comprising means providing a magnetic fluxand including an air gap, a conductive element movably mounted in said air gap and including a .tip adapted to cooperate with a phonograph record, a pair of conductive supporting members, and means conductivelycoupling saidmembers with said element, said last-named means constituting a conductive path for conducting to said supporting members voltages generated by the movement of said element in said airgap upon actuation thereof by the record.

ROELOF VERMEULEN. HERMANUS STEFANUS FUCH 

